1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring and controlling the quality of the galvannealed coating of steel strip immediately following the galvannealing step, wherein the hot galvanized coating of the steel strip produced by hot dipping is converted into FeZn phases by diffusion reactions through a thermal treatment in an annealing furnace at temperatures above the melting point of zinc.
2. Description of the Related Art
In order to improve the corrosion protection, the protection against wear, the surface hardness or the sliding behavior of metals, but also for reasons of a better appearance, it is known in the art to provide the metals with a coating with another metal, for example, to provide iron with a coating of zinc or chromium.
Various known methods have been found for applying the coating. They are:
Galvanic coatings, wherein the metal coating is applied in suitable baths containing acids or aqueous solutions by electrolysis of the metal immersed in the baths. When this method is used, coating thicknesses of up to about 10 .mu.m are achieved; PA1 Hot dip coatings, wherein the coatings are produced by immersing the metal to be coated in baths of liquid molten metal. In this method, a thin alloy layer is formed as a result of reactions between the metal atoms of the liquid coating metal and the atoms of the base metal, wherein a layer of pure coating metal is then placed on the alloy layer. The coating layer thicknesses achieved by this method are significantly higher than those achieved by galvanizing and are, for example, in the case of galvanizing iron (hot galvanizing) 4-50 .mu.m; an improvement of the hot dip coatings can be achieved by PA1 Galvannealing, wherein the term galvannealing is coined by combining the words galvanizing and annealing.
In the case of galvannealing, the steel strip which emerges from the molten metal bath and is now hot galvanized is thermally aftertreated by annealing at temperatures above the melting point of zinc. During this annealing step, diffusion processes take place which also penetrate through the upper layer composed of pure coating metal, i.e., zinc, so that different FeZn phases are formed.
When the galvannealing process is carried out in an optimum manner, the coating contains about 10-12% Fe. Higher Fe contents of greater than 12% Fe lead to an unfavorable deformation behavior of the steel strip, wherein increased powdering occurs during a later deformation.
In the case of lower Fe contents of less than 9% Fe, the coating is not yet completely alloyed through and residual zinc remains at the surface of the coating. This leads to an undesirable non-uniform appearance with inhomogeneous product properties.
In order to obtain a galvannealed coating which is reacted through in an optimum manner, the thermal treatment must be controlled with respect to the duration of the treatment and with respect to the temperature level to values which are adjusted to the layer thickness. Possible control variables for the duration of the treatment are the length of the annealing zone of the annealing furnace and the speed of the steel strip, and a possible control variable for the temperature is the temperature of the annealing furnace.
At the present time, monitoring and controlling the quality of the galvannealed coating is carried out on a large technical scale in such a way that the temperature of the steel strip above the galvannealing furnace is used as the parameter for the quality of the coating. However, this type of measurement, which is carried out without contact by means of pyrometers, has the problem that a change of the emission behavior of the strip surface at the moment of complete alloying of the coating and also with continuing formation of the alloy in the galvannealed state takes place. There is also the danger that an indicated temperature difference is not the result of an actual difference, but is caused by a changed emission. Consequently, a properly functioning control based on this measured temperature cannot be carried out satisfactorily.
Another disadvantage of the temperature measurement is the fact that the ideal temperature for reaching the desired galvannealed state depends on the reaction behavior of the steel, on the bearing weight, on the aluminum content in the zinc bath and on the temperatures during galvanizing. The galvannealing temperature mentioned above constitutes only an indirect measurement and must be considered in relation to the material used and the galvanizing conditions. The statement of an optimum galvannealed temperature can only always refer to a specific material and a specific combination of the galvanizing parameters which have an influence on the galvanizing process.
Another possibility for determining the quality of the galvannealed coating by measurement technology in order to carry out a control is the combination of temperature measurement and the Fe measurement at the cold measuring location. A disadvantage is the position of the measuring location which may be up to 100 m behind the galvannealing furnace and another disadvantage is the unsatisfactory accuracy of the iron measuring device. Also, it is not possible to recognize differences in the alloying condition over the strip width.